1. Field of the Invention
The present invention relates to conductive resin particles possessing a multilayer structure, and to anisotropic conductive adhesives based on the same which are useful in (1) the connection of LCD (Liquid Crystal Display) with its driving circuits, e.g. TCP (Tape CarrierPackage), FPC (Flexible Printed Circuit), (2) connection of semiconductor chips onto an LCD glass substrate named COG (Chip on Glass), (3) connection of semiconductor chips to a circuit substrate, for example in COF (Chip On Flexible printed circuit) and COB (Chip On Board), (4) connection of a semiconductor chip called FCA (Flip Chip Attachment) to a semiconductor substrate, and the like.
2. Description of the Prior Art
Anisotropic conductive adhesives are fundamentally composed of an adhesive resin and conductive particles being dispersed therein. The characteristic feature that such anisotropic conductive adhesives offers in making the connection lies in the anisotropy which permits electric current to be conducted only between the sites intended to be connected by way of conductive particles or in the direction of the Z-axis alone but not in the directions of the X- and Y-axes, while, on the other hand, the characteristic feature in terms of use is in that a extremely huge number of sites can be connected simultaneously.
The anisotropic conductive adhesives, ever since they were for the first time used in the form of ACF (Anisotropic Conductive Films) in the connection of liquid crystal panels for electronic calculators in the 1970""s, have been finding widened application in the liquid-crystal related industries as a connecting material with high reliability, and also have recently started to be utilized in other application field of semiconductor fabrication.
The anisotropic conductive adhesives have advanced steadily together with liquid crystals and also achieved higher function performance, while finding extensive application in the simultaneous multi connection between ITO (Indium Tin Oxide) tabs on a glass substrate for liquid crystals and printed contacts on its driving circuit substrate. During the course of this, great progresses have been made in (1) the size reduction of electronic parts as well as (2) the downsizing or the downscaling of connecting sites, coupled with the space narrowing between connecting sites, owing to multiplication of connecting sites brought about by full-colored and enlarged-liquid crystal display screens, whereby (1) correspondence to the space narrowing between connecting sites and (2) improvement of reliability of connection are consistently demanded for the anisotropic adhesives.
In the field of semiconductor packaging, on the other hand, there have been developed the packaging process by means of the flip chip connection in which a silicon chip is arranged in the face-down manner for the purpose of faster action of semiconductors. Referring to the flip chip connection, especially, the ACF-based technique makes it feasible to eliminate entirely the under-fill step to suppress stress concentration on the connecting sites caused by variations in coefficient of linear expansion between silicon chip and semiconductor substrate, and therefore has been attracting attention. Furthermore, the ACFs are a lead-free, environment-friendly material, unlike the C4 connection (Controlled Collapse Chip Connection) which is also one of the same flip chip connection procedures.
Because of the above-described characteristic features, the anisotropic conductive adhesives which once had been used only as an elementary material for connection of liquid crystals have become widely known in the semiconductor packaging field as well by following the currently prevailing trend toward size reduction and higher function performance including CSP (Chip Size Package, Chip Scale Package) and BGA (Ball Grid Array) in the industrial circles of semiconductors.
The anisotropic conductive adhesives are now used in large quantities with increasing amounts of liquid crystals used, and in recent years, their new application fields have been developed, such as resin film fabrication of liquid crystal panels and the above-mentioned application in the semiconductor packaging field. Namely, the anisotropic conductive adhesives have undergo changes in terms of quantity and quality, but the strongly demanded requirements for them have remained unchanged, lying in (1) correspondence to the space narrowing between connecting sites and (2) improvement of reliability of connection. How to cope with such demanded requirements depends largely upon the design of conductive fine particles in light of the connection mechanism of anisotropic conductive adhesives, especially the mechanism of how reliability of connection can be developed. In other words, the future promise of the anisotropic conductive adhesives could be influenced to a great extent by their key material, or the conductive particles.
The reliability of connection requires an understanding of the mechanism of how the anisotropic conductive adhesives conduct current. As illustrated in FIGS. 1 and 2, heating, followed by compression, allows the conductive particles to be located between the connecting sites to thereby achieve electrical connection.
In order to secure electrical connection, on the occasion of this, it is required to keep on compressing the connecting sites against the conductive particles. The continued compressing force arises in the first place from shrinkage through curing of the adhesive, whereby the stress is in a direct proportion to (1) elastic modulus of the adhesive, (2) xcex94T (difference between the curing temperature and the use temperature) and (3) xcex94xcex1 (difference in coefficient of linear expansion between the adhesive and the substrate to be joined). When the stress force becomes too great, warping takes place as is being described below, leading to deteriorated long-term reliability of the connection material. Conversely, too much small force fails to produce the acting force sufficiently to deform the conductive particles to a greater extent, resulting in increases in connection resistance, and is therefore not desirable.
Secondly, the continued compressing force is the force of repulsion against deformation of the conductive particles. Consequently, resin particles are preferred to metal particles such as nickel particles. In addition, such deformation broadens the contact area of the connecting site with the conductive particles, which in turn decreases electrical resistance and improves reliability of connection. Such conductive particles which have been investigated so far in the past include, for example, carbon particles such as particles of carbon black and graphite, metal particles such as particles of aluminum, nickel, copper, silver, gold, and resin particles having their surfaces covered with metal.
With reference to the resin particles having their surfaces covered with metal, furthermore, particles of insulating resins, such as polydivinylbenzene, crosslinked polystyrenes, crosslinked acrylic resins, benzoguanamine resins and melamin resins were investigated and have been put into actual use. However, it has been known that in the case of use of resin particles, there are encountered the following problems:
Namely the conductive particles remain in surface contact with the connecting sites while the used adhesive is heated and compressed, and in this case, it is preferable to secure the greater contact surface area, since it yields the smaller contact resistance. In addition, when the conductive particles show an increased restoring rate, they get into contact with the connecting sites under enhanced contact pressure, which in turn facilitates the contact resistance to be maintained at a constant level over a prolonged period of time. Nevertheless, there is incurred a contradiction in the facts that the more flexible conductive particles give rise to the greater contact surface area, whereas the more rigid conductive particles yield the higher restoring rate. In other words, when the conductive particles are made more flexible to decrease the contact resistance, they become more susceptible to plastic deformation and exhibit inferior elasticity and lower restoring rate, with the result that the contact resistance get less stable. Conversely, when the conductive particles are made more rigid, they show greater restoring rate and act to raise the contact pressure but at decreased contact surface area as small as near to the point contact, resulting in increased contact resistance; in either case, there is encountered the problem that the resultant electrical connection lacks in reliability.
Under these circumstances, it may be conceivable to provide the conductive particles having an intermediary degree each of flexibility and restoring rate, but in such a case, it is not possible to retain the advantages that they, with their flexibility, would be susceptible to deformation and that they, because of their rigidity, would develop an increased restoring rate, while making up for their individually derived disadvantages. This, coupled with their intermediary properties being just intermediate between the two states, leaves the problems unsolved in that their initial resistance is far from being low and unsatisfactory and that the resultant long-term reliability of electrical connection after aging is in adequate.
The conductive particles for the anisotropic conductive adhesives which can meet these contradicting properties simultaneously include, for example, multilayer-structured particles each having a rigid, highly restoring layer and a flexible, deformable layer.
Particularly, JP-A Hei 11-209714 discloses an art covering the conductive particles which are characterized in that such conductive particles are made of an acrylic resin consisting of a flexible core and a shell more rigid than the core. In this specification, mention is made only of a shell/core weight ratio as a factor influencing the restoring rate, but it should be pointed out that only if such weight ratio is resorted to, there would in some cases be produced conductive particles with inferior restorability and consequently deteriorated long-term reliability as an anisotropic conductive adhesive, depending upon the particle composition.
Also, JP-A Hei 8-193186 discloses a particle structure reverse to that of JP-A-Hei 11-209714, or an art covering the conductive particles which are characterized in that said conductive particles consist of a flexible outer layer and a inner core being more rigid than the outer layer. In this case, it should be noted that the said conductive particles, because of their flexible outer layers, often exhibit enhanced plasticity and therefore reduced elasticity, leading to inferior restoring rate, as compared with the counterparts having the same degree of modulus of elasticity or flexibility as those disclosed in JP-A-Hei 11-209714.
In an example of production multilayer particles, additionally, the description is given that multilayer and composite particles are produced with use of hybridization by allowing two kinds of particles to collide at high speeds. This signifies that the two layers exist independently, with absence of any chemical bond between them, often providing the multilayer particles with deteriorated restoring rate.
As a measure in connecting of FPC to a liquid crystal panel, JP-A-Hei 8-188760 discloses the conductive particles which are characterized by less than 10 kgf /mm2 in compressive strength at 10% compressive deformation. However, decreased compressive strength alone does not yield any anisotropic conductive adhesives with realizable long-term reliability, as mentioned previously in the above.
In consideration of the above-described problems, the object of present invention is to provide conductive multilayer structured particles having in combination the contradictory properties of flexibility and restoring property or restorability.
Also, another object of the present invention lies in providing the conductive multilayer structured resin particles and the anisotropic conductive adhesives containing such particles, which permit connection to be made under such a low pressure as may suppress development of cracking in the ITO tabs and also can realize increased stability of connection, especially long-term stability of connection.
The present inventors, with a specific aim to solving the above-described problems, conducted repeatedly extensive investigation, and as a result, found that the conductive resin particles obtained by providing a metal covering to each of multilayer-structured resin particles, which particles are characterized in that said particles each comprises at least one inner layer being more flexible than the outermost layer and that at least one of the adjacent two layers is chemically bound, exhibit flexibility and restoring property in combination. In addition, it was found that the anisotropic conductive adhesive produced by dispersing such conductive multilayer-structured resin particles into a resin component of adhesive can permit connection to be made under such a low pressure as may suppress development of cracking in the ITO tabs and also can realize enhanced degree of reliability of connection, especially long-term stability of connection, and such findings have culminated into completion of the present invention.
Namely, the present invention relates to:
(1) A conductive, multilayer-structured resin particle in which at least one inner layer is more flexible than the outermost layer and is chemically bound to at least one of the two adjacent layers, and the surface of the outermost layer is covered with a metal;
(2) A conductive, multilayer-structured resin particle as described above in (1), characterized in that the difference in glass transition temperature between the most flexible layer and the most rigid layer is not less than 20xc2x0 C.;
(3) A conductive, multilayer-structured resin particle as described above in (1) or (2), characterized in that at least one of the chemically bound, two adjacent layers contains a graft-polymerizable monomer;
(4) A conductive, multilayer-structured resin particle as described above in (1) to (3), characterized in that said conductive, multilayer-structured resin particle possesses a three-layer structure consisting of a rigid, central core layer, an intermediate layer which is more flexible than the central core and the outermost layer which is more rigid than the intermediate layer, with any adjacent two of these three layers being chemically bound;
(5) A conductive, multilayer-structured resin particles as described above in (1) to (4), characterized in that said conductive, multilayer-structured resin particle under stress of 10% deformation rate exhibits a compressive strength of less than 10 kgf/mm2;
(6) A conductive, multilayer-structured resin particle as described above in (1) to (5), characterized in that said conductive, multilayer-structured resin particle furthermore shows a restoring rate of 5 to 90%;
(7) An anisotropic conductive adhesive which comprises an adhesive resin component and a conductive, multilayer-structured resin particle as described above in (1) to (6);
(8) An anisotropic conductive adhesive as described above in (7), characterized in that an adhesive resin component contains particles showing rubber elasticity;
(9) An anisotropic conductive adhesive as described above in (7), characterized in that a particle showing rubber elasticity is a multilayer-structured particle consisting of not less than two layers.
(10) A stress relaxing agent, characterized in that said stress relaxing agent comprises a particle showing rubber elasticity as described above in (9).